Aircraft—also referred to as airplanes herein—use their main engines such as gas turbine, turbo-fan or turbo-prop engines for taxiing on the ground, usually on an airfield or a maneuvering area of an airport. As the main engines of airplanes are not designed to operate efficiently in a low power state, such as needed during the taxiing operation, such maneuvering of the aircraft consumes a lot of fuel. Also the main engines produce noise that increases the overall noise emission at an airport.
Alternatively, special vehicles, such as aviation tugs or pushers, may be used to drag or push airplanes for moving on the ground. However, since such special vehicles are expensive themselves as well as the employment thereof has to be paid for, and such vehicles are not available in large numbers at most airports, they are commonly only used for short distances, e.g. for the push-back operation from a gate. Thus, up to now, the main engines are still used for most of the taxiing, which causes above described disadvantages.
Solutions for taxiing of aircraft are already known. For instance, DE 10 2008 006 295 A1 discloses an electric motor mounted onto a running gear leg of an aircraft. The electric motor comprises a motor shaft parallel to the axle of the ground wheels of the running gear. The motor shaft can be moved axially to engage/disengage with the wheel structure of the aircraft running gear.
WO 2009/086804 A1 discloses a motor for driving the wheels of an airplane landing gear, particularly a nose landing gear, which is disposed in the base of the landing gear strut or is installed as a wheel hub motor in the wheel hub or rim. In order to eliminate a heavy and complex transmission between the motor and the wheel, the motor is configured as a hydraulic motor or electric motor and sup-plied with energy via the landing gear strut.
WO 95/29094 shows a system for driving the landing gear of an aircraft, in which both wheels of a nose gear are drivable through a differential gear assembly or, alternatively, at least one wheel of each main landing gear assembly is driven by an electric or hydraulic motor powered by the auxiliary power unit (APU) of the aircraft.
Although improvements could be achieved with the afore-mentioned approaches, it has been found that—especially for large commercial aircraft—these approaches did not yield satisfying results in terms of providing the necessary power to drive the aircraft without the help of the main engines. A particular aspect concerns to efficiently use the highly limited space available in the proximity of the ground wheels at the undercarriage of an airplane for such drives.
Several locations for installation of a drive unit to one or more ground wheels of the undercarriage of an airplane—also called running or landing gear—have been investigated. Basically, a drive unit may be installed at a mounting location inside or outside with respect to the undercarriage, i.e. seen by an observer from the undercarriage, of a respective ground wheel. The inside mounting location is preferable in terms of transmitting mechanical forces into the undercarriage and for establishing the required electrical, mechanical and any other connections. However, the integration of the drive unit at the inside mounting location is complex due to the very limited space. This particularly applies in cases where a brake unit is to be provided. The outside mounting location is preferable with regard to the space requirement. However, airplanes (e.g. airplanes used on short routes) are often equipped with a brake fan at the outboard mounting location, wherein as brake fan a blower unit or exhauster unit may be used, which produces an air flow for cooling the brake unit associated to the respective ground wheel. The brake fan is mandatory and may not be removed. In smaller aircraft having only one wheel associated with the leg of the undercarriage, the brake fan may also be installed on the same side as the brake unit, as e.g. shown in U.S. Pat. No. 6,615,958 B1.
Finally, yet importantly, the drive unit associated to one or more ground wheels itself needs active cooling, since such drive unit comprises a high power density required for moving the airplane by supplying torque to the associated ground wheel. Preferably, cooling of the drive unit is performed by air to avoid in-tegration of a cooling system using fluids such as water or any other coolant. That is to say, the brake unit and the drive unit both require air cooling. However, the exhausted hot air from the brake unit already has high temperature and thus, may not be used to cool the drive unit. Further the hot air from the brake unit is contaminated with brake dust may impair the drive unit when entering it.